1. Field of the Invention
The present invention relates to a test piece for measuring a substance in a body fluid, particularly in blood.
2. Prior Art
Heretofore, for the purpose of measuring a specific substance or substances in body fluids, especially in blood, it has been necessary to separate serum from blood and add a freshly prepared measuring reagent to this serum. Such operations were complicated and time-consuming. At the same time, if the blood or serum has been left for a long time before the start of analysis or if it takes time to prepare serum samples, the substances to be measured are often destroyed or changed and accurate quantitative analysis cannot be made. Further, many of the reagents used for measurements have a problem that they lack the stability after preparation, which forces a great waste of the reagents.
For solving these various problems, there have come into wide use the so-called test pieces in which reagents for detecting the intended substance are contained in an absorbent carrier such as filter paper. Among such test pieces, there is one in which the absorbent carrier impregnated with a measuring reagent is coated with an appropriate coating material so that when the coated carrier is brought into contact with blood, the substance to be measured separates from blood, i.e. the substance penetrates into the carrier leaving blood cells on the coated surface to permit the reaction with the measuring reagent. In such a test piece, the results of the reaction can be judged immediately after the reaction, by removing the blood cells on the carrier surface, through washing with water. Therefore, it enables the measurement in an extremely short time by using a small amount of blood without separating serum.
For example, in the case of diabetics, it is regarded therapeutically important to measure glucose in blood of the patient and determine the dosage of medicine to be administered based on the results of the measurement. This purpose may be achieved by using the aforesaid test piece.
As described above, it is considered that test pieces which permit the use of the whole blood as a specimen are desirable for rapid analysis. However, the technical difficulty in putting such test pieces into practical use lies in that no suitable coating material has heretofore been known which enables the swift and efficient separation of the substance to be measured from blood.
For example, there is known such technique that an absorbent carrier containing a measuring reagent is coated with a cellulose derivative such as ethylcellulose (U.S. Pat. Nos. 3,092,465 and 3,298,789). However, according to this method, strict selection is required with respect to the content of the substituents in the cellulose derivative and its viscosity in order to adjust the permeability of the substance to be measured (glucose), which is accompanied by a great difficulty from an aspect of preparing the coating materials. Further, the coated test pieces are usually cut into sizes suitable for actual use. But when the cellulose derivative is employed as the coating material, there is a problem that blood penetrates into the carrier through the cutting plane on measurement, and accordingly the results of the measurement tend to be deviated from the correct values.
On the other hand, it is also known a technique to employ a substance other than the cellulose derivative for the separation of a substance to be measured from the blood cells (Japanese Patent Publication No. 15669/1970). According to this method, the surface of a carrier containing a reagent for detecting the intended substance is coated with a hydrophobic substance such as fats and oils, wax, silicone etc. The disadvantage of this method is that since the hydrophobic properties of these substances are so strong that the permeability of the substance to be measured is excessively retarded and hence it takes unduly long time for measurement.
As the result of the intensive study for the purpose of solving the above-described problems, the present inventors have accomplished the present invention.